1. Field of the Invention
The present invention relates to a communication system including a plurality of interrogators and at least one transponder, wherein data transmission is effected between the interrogators and between each of the interrogators and the transponder. The invention also relates to the interrogator and transponder constituting the communication system.
2. Discussion of Related Art
There is known a communication system, as disclosed in JP-H06-68330A (publication of unexamined Japanese Patent Application laid open in 1994), including a plurality of interrogators which are disposed in proximity with each other and connected to each other via wires, and a host computer which is connected to the interrogators via wires so as to control the interrogators. In this communication system, each of the interrogators is arranged to detect and identify a moving body which is brought into proximity with the interrogator. There is also known a communication system, as disclosed in JP-2624815B2 (publication of Japanese Patent issued in 1997), including a plurality of interrogators and transponders, wherein the interrogators are arranged to communicate with the respective transponders by using respective different frequencies, so as to prevent interference of communications which could take place where the communications are simultaneously intended by the plurality of interrogators and transponders.
Further, there are known, as systems commonly used for communication between radio stations, FM (Frequency Modulation) system, AM (Amplitude Modulation) system, FSK (Frequency Shift Keying) and ASK (Amplitude Shift Keying) system. However, in recent years, OFDM (Orthogonal Frequency Division Multiplexing) system is coming into the limelight and put to practical use. This OFDM system is a multicarrier system in which transmission of data is performed with the signal being split into a plurality of sub-signals transmitted simultaneously by respective subcarriers, and is considered to have a high efficiency in utilization of frequency resources. An example of the OFDM system is disclosed in JP-H11-298438A (publication of unexamined Japanese Patent Application laid open in 1999).
However, where the interrogators of the communication system are connected via a wire network, a cumbersome operation is required to establish arrangement of the wiring in the system. The difficulty in the establishment of the wiring arrangement leads to a restriction in positioning of the interrogators and also to a difficulty in increasing the number of the interrogators in the system. Further, the presence of the large number of wires is not preferable for the system also in a sense that it could deteriorate an appearance of the system. For solving such problems, if the communication between the interrogators as well as the communication between the interrogator and transponder is intended in a wireless manner, there would be another problem that the communication between the interrogators and the communication between the interrogator and transponder might interfere with each other. Further, although the OFDM system is put to practical use in the field of high-speed digital communication owing to its high efficiency in utilization of frequency resources, there is still room for improvement in the efficiency of the system as a whole such as sharing of processing means with other systems.